Mobile telephone service using radio frequency transmission began here in the late 1970's. Initially, service was based on analog (FM) radio transmission for voice and Frequency Shift Keying (FSK) modulation, along with pre-assigned analog channels for control and signaling. ESS switching and standard trunking technology was utilized to provide access to and from the Public Switched Telephone Network (PSTN). This technology is commonly referred to as Advanced Analog Mobile Phone System (AMPS). A typical AMPS system consists of a Mobile Telephone Switching Office (MTSO) which controls a set of cell sites or base stations. The MTSO and cell sites communicate over standard voice trunks as well as dedicated control data links. The MTSO functions include standard local switching and call processing, radio channel management, location and handoff of mobile stations, billing, traffic measurements, service orders, trunk administration and system maintenance. The cell sites, on the other hand, provide for call set-up, supervision and termination, call handoff to/from adjacent cells as directed by the MTSO, protocol conversation from MTSO data links to an air (radio) interface, mobile location, remotely ordered testing, control and reconfiguration, RF (Radio Frequency) modulation/demodulation and transmission/reception and voice processing. An overview of AMPS is found in the Bell System Technical Journal, Vol. 58, No. 1, January 1979.
In 1987, through its working group, Groupe Special Mobile (GSM), the organization of the European PTTs (CEPT) adopted a new standard for a unified, pan-European digital cellular mobile radio communications system. This standard is commonly referred to as GSM. The basic building blocks of the GSM model include a network subsystem whose main component is a Mobile Switching Center (MSC) which is connected, on the one hand, to the PSTN and, on the other hand, to a base station subsystem. The base station subsystem comprises one or more base stations where each base station includes access to at least one Base Station Controller (BSC) and one or more Base Transceiver Stations (BTS). The BSC is responsible for performing cell management, channel management and intra-BSC handoffs. The network subsystem (NSS) is composed primarily of a standard ISDN switching system, called the Mobile Switching Center (MSC). The BSC interfaces with the MSC for implementing inter-BSC handoffs. The two main functions of the MSC are to act as a gateway into the PSTN and to interface with several mobile specific components of the network subsystem, including an authentication center, an equipment identity register and an operations and maintenance center.
In the United States, the Telecommunications Industry Association (TIA) is in the process of adopting a set of standards for migrating the current analog (AMPS) system to a dual mode operating environment in which a digital cellular network will co-exist with the present analog service. This collection of new standards is loosely referred to as AMPS-D. AMPS-D will subdivide existing air (radio channels) into six TDMA (Time Division Multiple Access) slots over which voice and control data will be transmitted. The .pi./4 DQPSK (Differential Quadrature Phase Shift Keying) modulation scheme will be used. The standard supports use of two time slots per frame to achieve full-rate coded voice transmission at approximately an 8 Kbps (Kilobits per Second) rate. Support for half-rate voice (4.8 Kbps) using a single TDMA slot per frame is also envisioned. Half rate voice will provide approximately a 6:1 overall capacity gain over AMPS. The reference model used for AMPS-D is a derivative of the GSM architecture. There are, however, some major differences in the operation of the two systems. The most significant difference lies in the air (radio) interface between the mobile terminal and the base station equipment. Unlike GSM, which uses a dedicated channel for signaling between the mobile and the BTS, AMPS-D uses control messages embedded within the traffic channels to perform call-related functions. To remain compatible with AMPS, though, a common analog channel is also used for the initial phase of call setup (until the type and capability of the mobile station is determined). The air (radio) interface for AMPS-D is documented in the TIA standard IS-54. These terms (AMPS-D and IS-54) thus refer to the same air (radio) interface.
Several manufacturers have formed consortia to provide GSM systems in time for deployment in the 1991 time frame. Among these is a group called European Cellular Radio Consortium, or simply, ECR. ECR is producing a number of GSM-compatible subsystems that can be packaged and marketed in various combinations, and in conjunction with each of the switching systems of the member companies.
Experience since the introduction of the mobile telephone service has revealed that an overriding difficulty with the service is the provision of sufficient capacity to meet the demand. This is the reason, for example for implementing the new IS-S4 standard, providing for a 6:1 capacity gain over conventional AMPS. Notwithstanding this improvement, it should be apparent that the ability to provide for additional capacity, over and above that provided in the IS-54, would be extremely valuable.
Techniques are available for exploiting the silent intervals in voice conversations to increase transmission capacity. These systems are referred to as Time Assigned Speech Interpolation (TASI), Digital Speech Interpolation (DSI) and by other terms. TASI was originally introduced to increase the capacity of underwater telephone transmission systems. Campanella, in "Digital Speech Interpolation", from the COMSAT Technical Review, Vol. 6, No. 1, Spring 1976, at pages 127 et seq., describes the use of these techniques in satellite communications. Others have proposed using these techniques in the mobile telephone service, see Al-Salihi, "The Application of Time Assignment Speech Interpolation (TASI) to the Radio Phone Service", from the 1980 Colloquium on Radio System Organization and Control; Riordon, "Demand Time-Assigned Speech Interpolation for Land Mobile Systems", from International Conference-Mobile Radio Systems and Techniques (1984); Nguyen, "Access Strategies for D-TASI Over Mobile Radio Channels", from the IEEE Vehicular Technology Conference (1986); and Sheikh, "Performance of Access Strategies for D-TASI Over Mobile Radio Fading Channels", from the Journal of the Institution of Electronic and Radio Engineers, Vol. 57, No. 6 (1987). More recently, Goodman, in "Packet Reservation Multiple Access for Local Wireless Communications", from the IEEE Transactions on Communications, Vol. 37, No. 8, August 1989, describes Packet Reservation Multiple Access (PRMA), which he indicates closely resembles TDMA. Goodman notes "[T]he task of implementing time assignment speech interpolation in a TDMA system with dispersed wireless terminals appears prohibitively complex."